In recent years, the miniaturization technology for electronic devices has been rapidly developed, and various kinds of portable electronic devices are becoming popular. Also, a battery, which is a power supply for these portable electronic devices, has been required to be miniaturized, and a nonaqueous electrolyte secondary battery having high energy density is attracting attention.
The nonaqueous electrolyte secondary battery obtained by using metallic lithium as a negative electrode active material is characterized in that the battery life is short because a dendritic crystal called dendrite precipitates on a negative electrode during charge although energy density is very high. Also, in this nonaqueous electrolyte secondary battery, dendrite can be grown so as to reach a positive electrode, thereby causing an internal short circuit, and there are problems in safety. Therefore, a carbon material capable of absorbing and desorbing lithium, specifically graphitic carbon, has been used as a negative electrode active material substituted for metallic lithium.
In order to increase the energy density of a nonaqueous electrolyte secondary battery, it has been attempted to use materials having large lithium absorption capacity and high density for a negative electrode active material. Examples of such materials include an amorphous chalcogen compound and elements such as silicon and tin which form an alloy with lithium. Among these materials, silicon can absorb lithium until the atomic ratio Li/Si of lithium atoms to silicon atoms reaches 4.4. Thus, the negative electrode capacity per mass of silicon is about 10 times as large as that of graphitic carbon.
However, the volume of silicon is greatly changed with the insertion and elimination of lithium during a charge and discharge cycle. This volume change of silicon results in the pulverization of a negative electrode active material particle, which causes the problem that the cycle life of the nonaqueous electrolyte secondary battery is deteriorated.